Homogeneous immobilization of simulated actinides in rhabdophane and comparison of its leaching stability with monazite

Rhabdophane LnPO 4 ·0.667H 2 O (Ln = La to Dy) is an important precipitation-enrichment barrier to isolate actinides from groundwater. However, the bearing capacity of different actinides in rhabdophane has been poorly understood due to the complex coordination environment induced by water molecules. In this work, the precipitation behaviors of Nd x Sm x La 1-2 x PO 4 ·0.667H 2 O ( x  = 0.1–0.5) solid solutions are systematically investigated to understand the simulated actinides occupancy and the chemical precipitation processes. The results show that Nd 3+ and Sm 3+ can be removed over 99% after 12 h in an approximate pH = 1 solution. Nd x Sm x La 1-2 x PO 4 ·0.667H 2 O single-phase has been observed by refined XRD , although the composition distribution of nanograins is extremely uneven. The distribution of simulated actinides can be homogeneous after prolonging reaction time to 12 days or sintering temperature over 1000 °C. Both Nd 3+ and Sm 3+ are preferentially incorporated on non-hydrated lattice sites when x  ≤ 0.1. Besides, the chemical stability of La 0.332 Nd 0.334 Sm 0.334 PO 4 ·0.667H 2 O rhabdophane is compared with associated monazite to understand the leaching activities of simulated actinides incorporated on hydrated and non-hydrated sites. The leaching rates of monazite are smaller than that of rhabdophane, suggesting that [LnO 9 ] polyhedrons in monazite have a stronger energy threshold than [LnO 8 ] and [LnO 8 ⋅H 2 O] polyhedrons in rhabdophane.